1. Field of the Invention
This invention relates to a flow volume control device suitable for use in a power steering system where an operating fluid delivered from a pump is fed to a power steering mechanism through a throttle passage and surplus fluid is refluxed to the suction side of the pump through a by-pass passage. More particularly, the invention concerns a flow volume control device which is adapted to lower the flow rate of the fluid to be supplied to a power steering mechanism in response to increases in the rotational speed of a pump.
2. Description of the Prior Art
In a high speed operation of a motor vehicle, it is desirable to let the driver perceive the reaction force of steering in an increased degree. For this purpose, there has been developed the so-called speed sensitive type pump which is adapted to reduce the discharge rate of an operating fluid to be fed to a power steering mechanism in response to increases in the rotational speed of the pump.
FIG. 1 shows an example of the flow volume control device with such an function, utilizing the pressure differential which is produced across a fixed orifice or throat 1 according to the increase of the discharge rate of a pump resulting from increases in its rotational speed. Namely, an auxiliary spool 2 is shifted by the pressure differential for variably controlling the open area of throttle passages 3 in such a manner as to lower the discharge rate Q when the rotational speed N of a pump reaches a predetermined value.
Since the open area of the fixed orifice 1 is constant, the conventional flow control device of the above-described construction has a problem that the pressure differential across the auxiliary spool 2 is increased abruptly by the increase of the rotational speed N of the pump as indicated at A3 of FIG. 2, suddenly shifting the position of the auziliary spool 2. Accordingly, the throttle passages 3 are constricted quickly and the discharge rate Q is varied from Q1 to Q2 as indicated A1 of FIG. 3, failing to give a smooth feeling in steering.